Typical gaseous reduction systems are disclosed, for example, in U.S. Pat. Nos. 3,748,120; 3,765,872; 3,905,806; 4,046,557; 4,099,962 and 4,150,972. These all teach the use of an external reformer to catalytically convert natural gas and steam into carbon monoxide and hydrogen for use as an iron ore reducing agent. This reformed gas is then passed through a bed of particulate ore at temperatures on the order of 760.degree. to 1,000.degree. C. to produce sponge iron.
It is well known that the hydrocarbons of natural gas per se have some reducing capacity, but are effective only at such high temperatures as to be impractical for direct use in any sponge iron process. In order for direct reduction by natural gas to be effective to give metallizations in excess of 80%, temperatures above 1000.degree. C. would be necessary, which would cause agglomeration. Such agglomeration of particles causes problems in solids flow through the reactor resulting in unacceptably uneven processing and often in complete blockage. See U.S. Pat. No. 4,268,303.
While reduction systems using natural gas reformed with H.sub.2 O or CO.sub.2 have been extensively used commercially, the catalytic reformers they employ are costly pieces of equipment and form a substantial part of the capital investment in a such sponge iron producing plant. They are also an additional source of heat loss.
U.S. Pat. No. 3,827,879 shows an even earlier attempt to practice gaseous direct reduction of iron ore With natural gas without any external reformer. However, this differs significantly in that it discloses a fixed bed process that uses methane injection alone (without steam addition), oxidized by partial combustion with oxygen (requiring separate combustion chambers) to obtain most of the hydrogen and carbon monoxide used for reduction.
Other patents such as U.S. Pat. Nos. 4,246,024; 4,253,867; and 4,261,734 have disclosed methods using gasifier gas or coke oven gas as a feed to a sponge iron reduction system which does not have an external reformer. However, in all the feed gases proposed for such systems, typically each already has H.sub.2 and/or CO as at least the major portion of such feed gas. For example, the gas composition of coke oven gas typically includes about 50% H.sub.2 and 7% CO in addition to about 30% CH.sub.4.In contrast, at least 95% of natural gas typically is composed of hydrocarbons. See Chemical Engineer's Handbook, John Perry ed. 4th edition, table 9-11.
It is, accordingly, an object of the present invention to provide a method for start-up of an iron ore reduction process which eliminates the need for an external reformer, a coking installation, a gasifier unit, or any other similar accessory equipment for generating effective reducing gas containing high percentages of H.sub.2 and CO, or for any pre-existing catalyst.
It is a further object of the invention to provide a method wherein hydrocarbon, gaseous at reducing temperatures (preferably in the form of natural gas) and water (preferably in the form of steam) are the only make-up needed for supplying the reduction process during both its start-up and its subsequent normal steady-state operation.
Other objects of the invention will be in part obvious and in part pointed out hereafter.
The aforementioned parent U.S. Pat. No. 4,528,030 provides a partial solution to some of the foregoing objectives, at least under steady-state conditions. That patent discloses that by adding a make-up of gasifiable hydrocarbon and H.sub.2 O (preferably as natural gas and steam) in the proper proportions to the already-established recycle reducing gas circulating through a bed of partially-reduced iron ore, metallizations of 90% or greater can be achieved at temperatures of 800.degree. to 1,000.degree. C. without any costly separate reformer equipment at all and without the formation of large agglomerates.
Lower investment and operating costs as well as fuel conservation can also be achieved by this elimination of an external reformer, because the heat losses and inefficiencies associated with such additional separate equipment are avoided.
The above-mentioned method uses the catalytic potential of the sponge iron to reform within the reducing reactor the gaseous hydrocarbons into a mixture of hydrogen and carbon monoxide using steam as the oxidizing agent. However, with such a process, during the start-up procedure no sponge iron will be available within the reactor. A possible solution could be to charge the reactor initially with sponge iron already produced somewhere else. This proposal is not usually feasible, and has the further disadvantage that the sponge iron is a material with weak mechanical characteristics and consequently a significant amount of fines will be produced during the charging procedure causing severe problems in the gas distribution through the bed and making impossible a smooth start-up. Another possible solution could be the utilization of an external source of reducing gas such as an external reformer to be used only during the start-up procedure to reduce the first batch of iron ore and produce the required sponge iron to initiate the in-situ reforming of gaseous hydrocarbons. The investment costs, however, of such a stand-by start-up reformer would be unacceptable. Finally, heating a cold bed of iron ore with a circulating stream of steam and natural gas fails to establish any effective reduction of the ore.
In unique recognition of this, applicants have developed a start-up method of great utility and technical advantage which is wholly independent of the use of pre-existing sponge iron or of any external source of effective reducing gas (i.e., H.sub.2 or CO).